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September 28, 2008

Generating Hydrogen Without a Carbon Footprint

by Sam Savage

By Shelley, Suzanne

NANOTECHNOLOGY A greener, less-expensive method to produce hydrogen fuel may eventually be possible using water with the help of solar energy and nanotube diodes that use the entire spectrum of the sun's energy, according to Penn State scientists.

"Other researchers have developed ways to produce hydrogen with mind-boggling efficiency, but their approaches have very high costs," says Craig A. Grimes, professor of electrical engineering. "We are working toward something that is cost-effective."

Most hydrogen today is produced via steam reforming of natural gas. As a fuel source, this presents two problems. The process uses natural gas and does not reduce reliance on fossil fuels. In addition, a byproduct is carbon dioxide, which creates a carbon footprint.

Grimes' process splits water into its two components, hydrogen and oxygen, and collects the products separately using commonly available titanium and copper. Splitting water for hydrogen production is an old and proven method, but in its conventional form, it requires previously generated electricity. Photolysis of water has also been explored, but is not yet a commercial reality.

Grimes and his team produce hydrogen using solar energy and two different groups of nanotubes in a photoelectrochemical diode. They report that using incident sunlight, "such photocorrosion-stable diodes generate a photocurrent of approximately 0.25 mA/cm^sup 2^, at a photoconversion efficiency of 0.30%."

"It seems that nanotube geometry is the best geometry for production of hydrogen from photolysis of water," says Grimes.

In Grimes' photoelectrochemical diode, one side is a nanotube array of electron donor (or n-type) material - titanium dioxide- and the other is a nanotube array with holes that accept electrons (a p- type material) - a mixture of cuprous oxide and titanium dioxide. P- type and n-type materials are common in the semiconductor industry. Grimes has been making n-type nanotube arrays by sputtering titanium onto a surface, anodizing the titanium with electricity to form titanium dioxide, and then annealing the material to form the nanotubes used in other solar applications. He makes the Cu^sub 2^O/ TiO^sub 2^ nanotube array in the same way and can alter the proportions of each metal.

Although TiO^sub 2^ is highly absorbent of the ultraviolet portion of the sun's spectrum, many p-type materials are unstable in sunlight and are damaged by ultraviolet light - i.e., they photocorrode. To solve this problem, the researchers made the TiO^sub 2^ side of the diode transparent to visible light by adding iron and exposed this side of the diode to natural sunlight. The TiO^sub 2^ nanotubes soak up the ultraviolet light between 300 and 400 nm. The light then passes to the Cu^sub 2^O/TiO^sub 2^ side of the diode, where visible light from 400 to 885 nm is absorbed, covering the light spectrum.

The photoelectrochemical diodes function in the same way that green leaves do, although not quite as well. They convert the energy from the sun into electrical energy, which then breaks up water molecules. The TiO^sub 2^ side of the diode produces oxygen and the Cu^sub 2^O/TiO^sub 2^, side produces hydrogen.

Although an efficiency of 0.30% is low, Grimes notes that this is just an initial result and that the diode can be readily optimized. "These devices are inexpensive, and because they are photo-stable, they could last for years. I believe that efficiencies of 5% to 10% are reasonable."

Grimes is currently working on a faster and easier electroplating method of manufacturing the nanotubes.

Photoelectrochemical nanotube diodes function like green leaves by converting energy from the sun into electrical energy, which then breaks up water molecules. Oxygen is produced on the TiO^sub 2^ side and hydrogen on the Cu^sub 2^O/TiO^sub 2^ side. Image courtesy of C. Grimes.